Constructing broken rock supports for roofs of cavities storing liquified hydrocarbon gases

ABSTRACT

Liquified natural gas, ethane or propane is stored in a cavity in which the roof is supported on broken rock placed in the lake. Very cheap broken rock containing a large volume of voids,when piled,is provided by building the cavity on a rock formation, breaking the rock formation into small pieces by rippers or giant teeth pulled by giant tractors, sieving the broken rock into fractions with each fraction having rock pieces with a minimum to maximum size ratio of 0.5, and piling the rock fractions without substantial mixing in the cavity. This allows storage cavities to be built at a fraction of the former cost and that also can not be damaged by sabotage and riots like former storage can be.

United States Patent St. Clair [451 Dec. 5, 1972 [5 41 CONSTRUCTINGBROKEN ROCK SUPPORTS FOR ROOFS OF CAVITIES STORING LIQUIFIED HYDROCARBONGASES [72] Inventor: John C. St. Clair, Box2l6, RR. 5,

London, Ohio 43140 [22] Filed: June 16, 1970 [21] Appl. No.: 46,829

OTHER PUBLICATIONS Micromeritics by J. M. Dallavalle, 2d Edit. PitmanPublishing Co. N.Y., N.Y. pp. 134-141 Primary Examiner-.lacob Shapiro[57] ABSTRACT Liquified natural gas, ethane or propane is stored in acavity in which the roof is supported on broken rock placed in the lake.Very cheap broken rock containing a large volume of voids,when piled,isprovided by building the cavity on a rock formation, breaking the rockformation into small pieces by rippers or giant teeth pulled by gianttractors, sieving the broken rock into fractions with each fractionhaving rock pieces with a minimum to maximum size ratio of 0.5, and pi1ing the rock fractions without substantial mixing in the cavity. Thisallows storage cavities to be built at a fraction of the former cost andthat also can not be damaged by sabotage and riots like former storagecan be.

8 Claims, 1 Drawing Figure PAIENTED 5 W3 3. 704. 593

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CONSTRUCTING BROKEN ROCK SU PORTS FOR noors F CAVITIES STORING LIQUIFIEDI-IYDROCARBON GASES Natural gas(which is a mixture of methane with someethane) and much of propane gas are mostly used in the winter time forthe heating of houses. However it is very desirable to keep thefacilities producing and transporting the preceding hydrocarbonsoperating at a constant rate the year around. Also, with the importationof liquified natural gas by ship on the east coast from countries thathave not been noted for political stability, it is highly desirable tokeep as large a supply of natural gas on hand as possible in case adistant country supplying the liquified natural gas should suspenddeliveries in case of political unrest. As a result there has been alarge amount of work by numerous companies on the storage of normallygaseous hydrocarbons as liquids in cavities.

When normally gaseous hydrocarbons are liquified by cooling to lowtemperatures they occupy a very small volume in the liquid form ascompared with the volume they occupy in the gaseous form. When aliquified gaseous hydrocarbon with a boiling point lower than thefreezing point of water is stored in a cavity at atmospheric pressurethe hydrocarbon will evaporate and cool the sides and the bottom of thecavity to below the-freezing point of water. If the cavity is made sothat water tends to flow into the-cavity, when a crack occurs in earthor rock of the sides or bottom of the cavity water will flow into thecrack. But since the sides of the crack are beiow the freezing point ofwater the water flowing into the crack will freeze and stop up theleaking crack.

The use of cavities to store liquified propane gas and liquified naturalgas has been long practiced in this country. It has been found a cavitycan be built very cheaply in large sizes. However for large cavities thecost for supporting an insulated roof for covering the cavity is high.Also all prior cavities are very easy to destroy by homemade dynamitebombs thrown on their roofs.

The drawing shows one form of the invention as would be preferred when aroof is provided for a small storage cavity. The drawing is taken up indetail at nearly the last of the specification.

In my invention I support the insulated roof of a cavity on top ofbroken rock placed in and filling the cavity. In this case theinsulation may just be 3 or 4 feet of leaves, chopped weeds, hay orstraw placed on top of the broken rock. 0r more preferably, wheresabotage by home made dynamite bombs is feared, the insulation may be atleast 6 to 8 feet of 0.5 inch maximum size rock on top of the regularliquid storage section rock of the storage cavity. Then a layer ofpolyethylene sheet is placed on top the insulation. Then the top of thepolyethylene sheet is covered with at least 2 feet of small broken rockto protect the roof from stray bullets and home made bombs. The roof issloped into gutters that drain off rain water.

I supply the broken rock to fill the cavity by first picking a site forthe storage cavity that is over a rock formation or very near to one. Ialso first see that the rock formation is rippable by rippers pulled bytractors.

Breaking up rock by ripping is a practice that has been used widely inrecent years in the construction of 2 highways and in mining; Inripping, giant teeth are pulled by big crawler type tractors through therock. It

- is normally best to use the biggest tractor available. It is alsopractical to use one enormous tractor pulling the ripper and as many assix enormous tractors in a line pushing the tractor pulling the ripper.

As available tractors have gotten larger it has become possible to ripall but the hardest rocks. Even the hardest rocks can frequently beripped if they are weathered or are first slightly broken by a smallcharge of explosive. Ripping is a well developed art and the cost of aspecific ripping job is about always possible to estimate in advance.About all the ripping done in highway construction is done by bidssubmitted in advance. Equipment has long been available to measure theseismic velocity, or the velocity of sound through the rock, which givesa good indication of whether a rock is rippable. Further details onripping can easily be obtained from the Kelly Products Division of CRC-Crose International, Inc., PO. Box 3227, Houston, Tex., The AmericanTractor Equipment Corp., 9131 San Leandro St., Oakland, Calif. or othermanufacturers of rippers. For the hardest rock I prefer to use thepatented ripper, with an adjustable ripper point angle, of the KellyProducts Division above mentioned.

I use rippers to break up the rock in all cases because of two reasons.First, breaking up rock by rippers costs 50 percent and sometimespercent less than breaking rock by drilling holes and blasting. Second,and of much more importance, is the fact that breaking a rock formationby ripping produces rock pieces of a much more uniform size thanblasting.

It is important that the mass of broken rock filling the storage cavityhave a high percentage of voids between the pieces of rock. This isbecause a lake of a given volume will hold more liquified natural gas orother hydrocarbon liquid if there is a higher percentage of voids in themass of broken rock. Also the amount of rock required per cubic foot ofliquified gaseous hydrocarbon varies greatly with possible percentagesof voids in the mass of broken rock. For instance dropping thepercentage of voids from 50 percent to 33 percent doubles the amount ofrock-that must be used to store a given volume of liquified gaseoushydrocarbon. A decrease to 20 percent voids multiplies the amount ofrock required by four.

The volume of broken rock required to store a given 'volume of liquifiedhydrocarbon gas is of importance since it determines the amount ofrefrigeration required to cool the broken rock filling a storage cavitydown to the temperature at which the cavity will operate in finaloperation. In case of liquified hydrocarbon gases boiling attemperatures not so far below the freezing point of water, as forexample liquid propane, the refrigeration required to cool the brokenrock down to operating temperatures is of minor importance. Also in thecase where liquified natural gas is imported by ship the liquifiednatural gas has to be vaporized and heated anyway and the refrigerationthus obtained is very conveniently used to cool the broken rock fillingthe cavity. In this latter case for a broken rock mass containing 50voids it will take the evaporation of about half of the storage capacitybetween the voids of a cavity to cool the broken rock down to operatingtemperatures. In the case of liquified natural gas storage used to storenatural gas, during the summer, from a gas pipe line, cooling the brokenrock is more expensive. However, it is not as expensive as firstthought. In this latter case there will have to be refrigerationequipment at the cavity to cool the pipe line gas and liquify it fromthe pipe line that is used only part of the time. This equipment can beused the rest of the time to liquify natural gas that is evaporated incooling the broken rock in the cavity the electric power used will begenerated by the electric power company with the portion of boiler andgenerating equipment that has to be kept in reserve for supplyingelectricity to the motors at the storage cavity. Or in the language ofelectric power companies the amount of electricity used in the initialcooling of the cavity increases the amount of electricityused but doesnot increase the maximum demand of electricity during any second oftime. Therefore the only additional expense for the electric powercompany is the use of more fuel for boilers that are kept operating moresteadily. Electric power companies greatly like this and always givemuch cheaper rates for the additional electricity used.

However it is emphasized that the total amount of refrigeration percubic foot of liquid stored required to cool a cavity containing brokenrock prepared according to my invention will normally be less than thatfor storage cavities that have been constructed by prior methods. Thesecavities now in use made by prior methods are limited in diameterbecause of the necessity of supporting the roofs from the sides. Howeverthe diameter of my cavity containing broken rock is only limited by theamount of storage needed at a given location and this is normally verylarge.As a result my cavities containing broken rock usually will bebuilt in diameters of over=l,000 feet as compared with the storagecavities of perhaps 200 feet diameter now built. Therefore my storagecavities will have heat pick up from the sides of onlya small fractionof the heat pick up from the sides that a smaller diameter storagecavity will have that must support the roof from the sides. Therefore abroken rock filled cavity will normally require less total refrigerationper cubic foot of liquid stored as compared with presently used storagecavities through the refrigeration required by a rock filled cavity willhave a greater percentage of the refrigeration needed at the start.

It is the essence of this invention that the rock pieces made not onlybe made cheaply but more particularly they must provide in the cavity amass that contains a large percentage of voids.

l have found that the way to increase the percentage of voids is toincrease the ratio of the size of the minimum sized pieces of rock tothat of the maximum sized pieces of rock in any given part of the piledrock filling the cavity. This fact has been taken from the bookentitled'Micromeritics by J.M.Dallavalle, Pitman Publishing Co., N.Y.,2nd edition, pages 135-143. While the interest of other workers has beenin getting mixtures of broken rock with the minimum volume of voids,which is of extreme value in making strong and cheap concrete from amaximum of broken. rock and sand anda minimum of cement, the same datacan be used to determine what size ranges mixed together give the mostvoids. The data show that increasing the ratio of the maximum dimension,of the minimum sized in predicting the actual void volume that will beobtained in actual practice it is known that the fractions of voids in apile that are possible by mixing spheres of the same diameter arebetween 26.95 percent and 47.64 percent voids depending on how muchshaking and tamping are applied on the mass of spheres. (See referenceby Dallavalle previously quoted page 127.) However with broken rock theindividual pieces are not spherical but have angular corners that stickout and prevent the individual pieces from coming together as closely asspheres. This increases the volume of voids in a pile of broken rockpieces. The exact voidage on breaking a rock and sieving it to sizedfractions will not only depend much on the range of sizes in eachfraction(as can be predicted from Dallavalles Data) but also on theamount of corners on each rock piece and the amount of tamping. Anexample of the voids in piles of broken rock is given by J. J. Barker,Industrial Engineering Chemistry, Vol. 57, pages 46-47 where for pilesof granular broken basalt pieces ranging from 0.08 to 0.24 inches insize the voidage was measured as ranging between 41.3 percent and 44.5percent. Since this rock mixture has a ratio of minimum sized pieces tomaximum sized pieces of 0.33 it can be easily estimated from DallavallesData that limiting the ratio,of the dimensions of the minimum sizedpieces to the maximum sized pieces, to 0.5 that the voidage will beincreased by about 4 percent or the total voidage will be in the rangeof 45 percent to 49 percent. Dallavalle on page 144 shows that for rockpieces of this size and larger it is only the relative size to eachother of the rock pieces in a fraction and not the size of the rockpieces themselves that causes variation in voidage, assuming a constantamountof tamping. Therefore the above example is applicable incalculating what voidage fractions of larger rock pieces will have. Itmay be pointed out that Dallavale on page 137 shows that with very smallratios of minimum sized rock pieces to that of the maximum sizedpieces(as for example 0.001 which is not uncommon in ordinary mixturesthat are not sieved) that the voidage may go down to as low as 10percent which would make the use of it impractical to use to fill astorage cavity with.

In ,making the broken rock pieces filling a storage cavity it is aboutalways possible to examine a large number of prospective sites for anygiven cavity. Natural gas or liquid propane or other liquifiedhydrocarbons can be very cheaply transported as much as 25 to 50 milesand sometimes more from a storage place to where they are to be used. Asa result areas of over several thousand square miles can be carefullylooked over for the best site for a cavity. In my case the best 10-cation will also mean close proximity to a formation of rock that can beconveniently broken by ripping into pieces that can be used to fill thecavity and act as supports for the roof.

The ripping of the rock will be done by tractors pulling giant teeththrough the rock formation. A bulldozer with a blade in front will pushthe broken rock to a transportable set of sieves where the broken rockwill be sieved into sized fractions. The number of fractions, that canbe sieved economically, is large and as many as ten different sizedfractions can be made. This allows size ratios, of the maximum sizedpieces to the minimum sized pieces in a fraction, tobe maintained easilyover 0.5 though this is not necessary. The sized fraction with thesmallest rock pieces is reserved for building roads, over the partiallyfilled cavity, for hauling the broken rock to where it is dumped. Sincecavities of at least 75 feet depth to a maximum of 200 feet depth willusually be built the roads will not have to be built very close togetherfor broken rock when dumped will roll quite a distance when dumped downa slope. However it is highly desirable to build roads of finer materialsince it allows large dump trucks with rubber tires to be used withoutheavy wear on the tires. In all cases the different-sized fractions ofrock are placed in separate and different positions in the storage lake.Intermixing of touching different-sized fractions is easily avoided ascan be seen from work done in the petroleum field using gravel at thebottom of oil wells to keep sand from being pumped up with the oil.There it has been found that gravel that is not over eight times thesize of the percent size of the sand will hold the sand back. The 10percent size of a sand is the size of the sand in which exactly 10percent of the total sand has a greater diameter than the 10 percentsize. (See Petroleum Production Engineering, Oil Field Development, byLC. Uren, 4th Ed., 1956, McGraw-Hill Book Co., New York City, page 718.)

In the final stage of filling a storage cavity with broken rock, a verysmall bulldozer pushing a blade is used to grade the top of the pile ofrock. If leaves, chopped weeds or hay or straw are used for insulationthey are put on. The roof itself is very conveniently made out of asheet of polyethylene plastic covered with a layer of broken rock orearth to protect against stray rifle bullets or home made bombs.

Referring to the drawing, a cavity has been dug out of rock or earth ofthe original location as shown at 11. The rock that has been broken byripping and is used to support the roof has been sieved into fourfractions. The first of the four fractions is the largest fraction andhas been placed at 8 on the right side of the cavity in the drawing. Thesecond largest fraction of the broken rock has been placed at 9 in thecenter of the cavity as shown in the drawing. The next smallest fractionof the broken rock has been placed at 10 as shown in the drawing. Thesmallest fraction of the broken rock, or the fines, has been placed at 6and 7 as shown in the drawing. As has been described previously thefines 6 and 7 have been used to slowly build up roads duringconstruction on which trucks can haul the other fractions of broken rockand can dump these other fractions from. Insulation, in this case,consists of a layer of rock fines 5 placed on top of the fractions ofrock 6, 7,

8, 9, and 10. On top of the insulation 5 is placed a plastic sheet 4. Ontop of the plastic sheet 4 is placed a layer of unsieved rock to preventdamage to the plastic sheet 4 by stray bullets or from actualintentional sabotage. The liquified hydrocarbon gas is introduced orremoved by pipe 1 with the aid if required of a pump not shown. Thestorage cavity is vented by vent pipe 2 which is connected if need be tomeans not shown for recovering hydrocarbon gases in the vented gases ifrequired. Drainage of rainwater from the top of plastic sheet 4 may bepreformed by means previously described but in-the drawing rain water isremoved by occassionally sucking it off by inserting a hose down to theaccumulated water.

In conclusion I may say that this application discloses a method forsupporting the roof of a storage cavity for liquified hydrocarbon gaseswhose boiling points, at any possible barometric pressure at thelocation of the storage cavity, are lower than the freezing point ofpure water, that costs only a fraction of prior means. Moreover thefinal storage cavities made possible by this patent are so much safer.In these days when arson, riots and sabotage have been so frequentlyoccurring the possibility of some saboteur throwing a homemade bomb ofdynamite on the roof of a storage cavity storing liquified natural gasis frightening. With former methods of supporting roofs for such storagethe roofs must be made light and would offer little resistance to a'dynamite blast. The blast would start off a fire and only extreme luckwould prevent all the liquified natural gas from burning with a veryspectacular fire.

By timing the sabotage and burning of a citys supply of stored naturalgas just before the coldest time of the winter the city would only haveabout a third of the needed gas per day to keep the houses of the peoplewarm. This would not be enough gas to keep the water pipes of all thehomes in the city from freezing and bursting. The amount of sufferingthat can be caused by a few home made dynamite bombs on the roofs ofprior types of liquified natural gas storage cavities is frighten-However with my method of supporting a roof for a storage cavity, a roofcan be covered with a thick layer of rock or earth and be madecompletely resistant to home made, dynamite bombs. If the saboteurs madeand used a super bomb only the natural gas at the top of the storagecavity would burn and the great amount of the liquified gas stored wouldstay cold and not burn because of the protection of the broken rockabout it.

Iclaim:

l. A method for constructing the supports for the roof for a storagecavity storing a liquified hydrocarbon gas with a boiling point, at thelowest possible barometric pressure of the locality where the cavity islocated, lower than the freezing point of pure water which comprises:ripping rock into smaller pieces, sieving the broken rock into at leasttwo different sized fractions, piling the mentioned rock fractions inthe storage cavity but with each fraction in different places from theother rock fraction, so that liquified hydrocarbons may be stored in thevoids between the pieces of broken rock, and supporting the roof of thestorage cavity on top of the broken rock placed in the cavity.

2. A method according to claim 1 in which the rock is ripped at alocation less than 5 miles from a border of the completed cavity.

3. A method according to claim 1 in which the rock is ripped at alocation less than one mile from a border of the completed cavity.

4. A method according to claim 1 in which the rock is ripped from alocation over which the completed cavity is finally built.

7. A method according to claim 5 in which the rock is ripped at alocation less than one mile from a border of the completed cavity.

8. A method according to claim 5 in which the rock is ripped from alocation over which the completed cavity is finally built.

1. A method for constructing the supports for the roof for a storagecavity storing a liquified hydrocarbon gas with a boiling point, at thelowest possible barometric pressure of the locality where the cavity islocated, lower than the freezing point of pure water which comprises:ripping rock into smaller pieces, sieving the broken rock into at leasttwo different sized fractions, piling the mentioned rock fractions inthe storage cavity but with each fraction in different places from theother rock fraction, so that liquified hydrocarbons may be stored in thevoids between the pieces of broken rock, and supporting the roof of thestorage cavity on top of the broken rock placed in the cavity.
 2. Amethod according to claim 1 in which the rock is ripped at a locationless than 5 miles from a border of the completed cavity.
 3. A methodaccording to claim 1 in which the rock is ripped at a location less thanone mile from a border of the completed cavity.
 4. A method according toclaim 1 in which the rock is ripped from a location over which thecompleted cavity is finally built.
 5. A method according to claim 1 inwhich, for a rock fraction, the average maximum dimension on a weightbasis of the minimum sized pieces is over 0.4 times the size of theaverage maximum dimension of the maximum sized pieces taken on a weightbasis.
 6. A method according to claim 5 in which the rock is ripped at alocation less than 5 miles from a border of the completed cavity.
 7. Amethod according to claim 5 in which the rock is ripped at a locationless than one mile from a border of the completed cavity.
 8. A methodaccording to claim 5 in which the rock is ripped from a location overwhich the completed cavity is finally built.